In general, a light emitting device is a compound semiconductor having a characteristic of converting electrical energy into light energy. The light emitting device may include compound semiconductors belonging to group III and V on the periodic table. The light emitting device can represent various colors by adjusting the composition ratio of the compound semiconductors.
When forward voltage is applied to the light emitting device, electrons of an N layer are combined with holes of a P layer, so that energy corresponding to band gap energy between a conduction band and a valance band may be diverged. The energy is mainly emitted in the form of heat or light. In the case of the light emitting device, the energy is diverged in the form of light. For example, a nitride semiconductor represents high thermal stability and wide band gap energy so that the nitride semiconductor has been spotlighted in the field of optical devices and high-power electronic devices. In particular, blue, green, and ultraviolet (UV) light emitting devices employing the nitride semiconductor have already been commercialized and extensively used.
A conventional ultraviolet light emitting device is formed by sequentially laminating a first conductive semiconductor layer, an active layer, and a second conductive semiconductor layer on a substrate, in which a hole injection layer is provided between the active layer and the second conductive semiconductor layer to allow holes to be smoothly flow.
The hole injection layer is mainly a single layer formed of GaN. However, in a specific ultraviolet region, for example, a band having a wavelength range of 240 nm to 300 nm, the hole injection layer functions as a light absorber, thereby reducing the optical efficiency.